I tried the Dolphin Engine on our performance test and it only scored 12. I sent them an email to see if they could improve it.
Chrome for iOS is an interesting one - it scores around 31-33 on our iPad 2, which is better than PhoneGap, but worse than Safari. Chrome for iOS has a slower javascript engine like PhoneGap, but I think it has hardware acceleration, which explains the result. It was perfectly playable so it seems promising.
Meanwhile Chrome for Android has disabled GPU acceleration on my HTC Sensation. Argh! So Chrome is now faster on iOS than on Android!